Square pegs, round holes and slow desorption

Predicting the transport and fate of hydrophobic organic contaminants in underground aquifers requires a mechanistic understanding of sediment-contaminant interactions. A rapid, pressure induced formation of a desorption resistant fraction of trichloroethylene (TCE) on three model adsorbents, i.e., silica gel, hydrophobic Y-zeolite, and polyvinyl chloride beads, was studied. Quantum mechanical calculations were performed to determine the energy associated with distorting the bond angles in a TCE molecule from their equilibrium configuration. Atomistic simulations were performed to determine the effect of separation distance on intermolecular repulsions between a TCE molecule and a silica surface. The effect of pressure on TCE adsorption was small or negligible after the first pressure step. TCE adsorption and desorption on this silica gel were completely reversible for short equilibration times, but a desorption resistant fraction could be induced by equilibration times longer than 1 day. The TCE pressure induced adsorption occurred in pores smaller than the effective size of a TCE molecule at 1 bar. The energies associated with even small molecular distortions could lead to high activation energies for desorption. Increasing contaminant sequestration with increasing contact time might be attributed to the activation energy required for penetrating pores smaller than the equilibrium size of the adsorbate molecule. The rapid development of desorption resistant TCE on the silica gel suggested that the diffusional distances associated with this mechanisms were small.